PAT 甲级【1013 Battle Over Cities】

1. 本题就是dfs.连通图个数-2；
2. 但是java慢，最后一个case 超时

import java.io.*;
import java.util.HashSet;
import java.util.Set;

public class Main {
    @SuppressWarnings("uncheck")
    public static void main(String[] args) throws IOException {
        StreamTokenizer st = new StreamTokenizer(new InputStreamReader(System.in));
        int n, m, k;
        st.nextToken();
        n = (int) st.nval;
        st.nextToken();
        m = (int) st.nval;
        st.nextToken();
        k = (int) st.nval;
        Set<Integer>[] g = new HashSet[n + 1];
        for (int i = 0; i <= n; i++) {
            g[i] = new HashSet<>();
        }
        for (int i = 0; i < m; i++) {
            st.nextToken();
            int city1 = (int) st.nval;
            st.nextToken();
            int city2 = (int) st.nval;
            g[city1].add(city2);
            g[city2].add(city1);
        }
        PrintWriter out = new PrintWriter(new OutputStreamWriter(System.out));
        for (int i = 0; i < k; i++) {
            st.nextToken();
            int city = (int) st.nval;
            Set<Integer> connected = new HashSet<>(g[city]);
            visited = new boolean[n + 1];
            g[city] = new HashSet<>();
            for (int connectedcity : connected) {
                g[connectedcity].remove(city);
            }

            int cnt = travel(g, n);

            out.println(cnt - 2);

            for (int connectedcity : connected) {
                g[connectedcity].add(city);
            }
            g[city] = connected;
        }
        out.flush();
    }

    static boolean[] visited;

    public static int travel(Set<Integer>[] g, int n) {
        int count = 0;
        for (int i = 1; i <= n; i++) {
            if (!visited[i]) {
                travel(i, g);
                count++;
            }
        }
        return count;
    }

    public static void travel(int start, Set<Integer>[] g) {
        visited[start] = true;
        for (int city : g[start]) {
            if (!visited[city]) {
                travel(city, g);
            }
        }
    }
}